U.S. Pat. No. 3,299,125 suggests a catalyst system for production of aromatic diacids which utilizes combination of two metals, one being cobalt, and a second being from a group which includes zirconium. This catalyst system is shown to be effective for oxidation of para-xylene to terephthalic acid at temperatures in the preferred range of 80.degree. C. to 130.degree. C. This temperature range is considerably lower than that necessary using other catalyst systems. The oxidation is performed in a solvent such as acetic acid and water, and oxygen or air is bubbled through the reaction medium. At 120.degree. C., reaction times of 8 hours are necessary to achieve conversions of para-xylene to terephthalic acid of sixty to eighty percent. With the combination of cobalt and zirconium as the catalyst, the highest yield of terephthalic acid shown was 88%. This patent describes the advantages of the catalyst system as being the ability to eliminate halide promoters from the catalyst system, thus significantly reducing metallurgy requirements in the reactor system.
U.S. Pat. No. 3,700,731 suggests a cobalt-catalyzed oxidation of para-xylene wherein the reaction is performed in a continuous stirred tank reactor with hold-up times on the order of 240 minutes, with a constant withdraw of product and recycle of unreacted feed and partially oxidized intermediates. The pressure of the reaction system is such that all of the reactants are kept in the liquid phase, and air or air enriched with oxygen up to 50% is preferred as the oxidant. The product removed from the reactor is highly impure, with terephthalic acid crystal purities of about 85%, and a large quantity of partially oxidized intermediates dissolved in the liquid phase. Terephthalic acid is removed from the reactor product by filtering crystals from a cooled reactor product, and then recycling the liquid back to the reactor. The filtered crystals are "digested" by contacting with a solvent such as acetic acid at a temperature of about 200.degree. C. to 300.degree. C. for about ten minutes or longer, and then filtration of the digested crystals at about 100.degree. C. and washing with hot water or acetic acid. These process operations result in a rather large recycle stream of partially oxidized intermediates to the oxidation reactor. A final terephthalic acid product is said to be 98 to 99% pure with a yield based on the starting para-xylene of 95% (molar). While the oxidation of para-xylene to terephthalic acid is highly exothermic, this patent is silent about how the oxidation reaction is cooled. Although adequate cooling of the stirred tank reactor can be achieved in a laboratory environment, it is a significant factor in design of the commercial reaction system because the produced terephthalic acid is sparingly soluble in the reaction solution. Crystals can therefore precipitate on heat exchange surfaces if temperatures of the heat exchange surfaces are significantly less than the temperature of the reaction mixture, resulting in ineffective heat removal.
The vast majority of commercially available terephthalic acid is produced by improved versions of U.S. Pat. No. 2,833,816, which suggests a catalyst combination of cobalt and manganese salts and a halide promoter, for example bromine. In this system, para-xylene is contacted with air in an acetic acid medium at temperatures in the range of 170.degree. C. to 210.degree. C., and so-called "crude" terephthalic acid is produced. Over time, these systems have been improved to the point where typical crude terephthalic acid purities of 98-99.5% are produced in yields of roughly 95-96% molar based on para-xylene feed with oxidizer contact times of 45-90 minutes. However, this system suffers from significant acetic acid decomposition in the range of 5-10 lb/100 lb of terephthalic acid produced. In addition, the acetic acid and halide promoters are highly corrosive, necessitating the use of higher metallurgy as the material of contact, namely titanium. The contributions of acetic acid losses and titanium metallurgy significantly increase manufacturing costs.
U.S. Pat. No. 5,523,474 suggests a catalyzed system with bromine promotion for para-xylene oxidation in an acetic acid medium. The reactor design is a so-called liquid oxygen reactor (LOR) which utilizes oxygen-enriched air in purities of 50-100% oxygen by volume. The benefits claimed include lower acetic acid decomposition and reduction in premature reactor shutdowns. While the patent describes the bromine-promoted system which requires titanium metallurgy, it fails to address the significant flammability concern associated with the incompatibility of oxygen-enriched air and titanium as is well known in industry and indicated in National Fire Protection Association 53 Guide on Fire Hazards in Oxygen-Enriched Atmospheres 1994 Edition.
For each of the oxidation techniques discussed above, after isolation the resulting terephthalic acid solids are generally referred to as "crude" terephthalic acid. The crude terephthalic acid can be up to 99.5% purity, with the primary impurities being 4-carboxybenzaldehyde (hereafter referred to as 4-CBA), para-toluic acid (hereafter referred to as pTA), and color containing species. Such a product is not of sufficient purity to be used directly for polyester fiber or bottle resins without additional purification, most typically by hydrogenation and re-crystallization. An example of such a purification technique is U.S. Pat. No. 3,584,039.
There is also significant commercial interest in production of isophthalic acid by oxidation of meta-xylene. Isophthalic acid is typically produced by processes similar to those used for terephthalic acid.
It would be desirable to have a process to produce these dicarboxylic aromatics wherein high yields and conversions are obtainable, wherein titanium equipment is not required, and wherein solvent decomposition is decreased. It is therefore an object of the present invention to provide an improved process for production of aromatic diacids having high yields and conversions, high crystal purities and reduced solvent decomposition losses.